The optimal host response to microbial pathogens requires balancing effective pathogen killing with limiting tissue pathology caused by the pathogen or by the host?s own immune response. This host disease tolerance phenomenon is especially critical in infections with helminths, which are macroparasites that can cause severe tissue damage and inflammation. Using a mouse model of hookworm infection with Nippostrongylus brasiliensis (Nb), we identify Resistin-like molecule (RELM)a as a highly secreted protein that protects the host from potentially fatal infection-induced lung tissue damage at the expense of optimal hookworm killing. Our central hypothesis is that RELMa is a host disease tolerance mechanism that shifts the balance from helminth killing to resolution of inflammation and tissue healing. RELMa is expressed by immune cells such as macrophages and non-immune cells such as epithelial cells (EC). In preliminary data utilizing bone marrow chimeras and macrophage co-cultures, we identified that RELMa- expressing alternatively activated macrophages (AAMac) are less efficient at Nb killing, but instead dampen lung inflammation and promote tissue repair. Further, we generate unique Arginase1/RELMa AAMac dual reporters, that reveal AAMac heterogeneity and implicate RELMa+ AAMacs as a new wound healing macrophage subset. Based on these findings, the focus of this proposal is to combine novel cell-specific RELMa KO/reporter mice with functional co-culture assays and new RELMa reagents to delineate RELMa function in macrophage- helminth interactions and mucosal tissue healing. In Aim 1, we will employ cell-specific RELMa KO/reporter mice and adoptive cell transfers to delineate the contribution of RELMa derived from innate cells (AAMac or eosinophils) or EC to Nb immunity and tissue healing. In Aim 2, we will investigate RELMa regulation of macrophage-Nb interaction using AAMac dual reporter mice, RELMa fusion proteins and blocking antibodies, and optimized co-culture assays. In Aim 3, we will employ 3D lung scaffold and EC air-liquid interface co-cultures with AAMacs and mesenchymal stem cells to determine how RELMa-expressing AAMacs and ECs interact with the lung stroma and aid lung tissue recovery. We anticipate that a better understanding of the beneficial versus pathogenic effects of RELMa in helminth infection could guide therapeutic strategies to enhance anti-helminth immunity while limiting pathologic inflammation and promoting tissue healing. Our findings provide new insight into alternatively activated macrophage biology and macrophage-stromal cell interactions, which could be broadly applicable to resolving mucosal tissue injury and inflammation that are of significant public health concern.